Conventionally, an air filter, e.g., a dust collector filter or the like, has been used widely for improving the labor environments in factories such as metal cutting factories or factories where powder dust is generated in a large amount, or for recovering relatively expensive powder. A polyester filament nonwoven fabric, which serves as a general-purpose polyester filament nonwoven fabric, and a polyester staple nonwoven fabric have been used as a filter base material thereof.
A bag filter obtained by sewing and forming a nonwoven fabric produced from staple fibers as a raw material into a cylindrical shape has been used so far as a filter base material for a dust collector; however in order to obtain sufficient filtration throughput, the cylindrical bag filter inevitably requires enlargement. Accordingly, in order to save the space and widen the filtration surface area, a filter using a pleated nonwoven fabric as a filter base material has been used. For this pleated-type filter, a nonwoven fabric with high rigidity is required so as to bear a load during filtration.
In order to obtain a nonwoven fabric with high rigidity, it is necessary to heighten the cross-sectional shape moment of the nonwoven fabric, that is, to make the thickness of the nonwoven fabric large. However, when the thickness is made large while lowering the fiber density, the network structure of the fibers is coarse, so that rigidity is not exerted. Consequently, a technique for making the thickness large by increasing its basis weight has been employed.
Regarding a polyester filament nonwoven fabric, in the case of using fibers in an oriented crystallization state by high speed spinning, a high basis weight causes insufficient fiber fusion by embossing, low elongation, high rigidity and insufficient moldability, so that there is a problem of deterioration in shape retention property.
Accordingly, in order to improve the fiber fusion property, there is proposed a method in which undrawn yarns that are not subjected to oriented crystallization are used as a thermobonding component, followed by embossing (e.g., see Patent Literature 1). However, in this method, since crystallization is promoted for fibers with low orientation degree by embossing, the fibers become brittle and there is a problem of deterioration in shape retention property and durability.
As a method for improving the rigidity and pleat retention property of a nonwoven fabric, there are proposed many methods using thermobonding components. There are proposed methods embossing a sheath-core type composite filament nonwoven fabric containing the low melting point component as a sheath component by compression bonding to form a shallow unevenness and suppress fuzz, thereby obtaining a nonwoven fabric having a bending resistance of 500 mg or more and a good pleat retention property (e.g., see Patent Literatures 2 and 3).
There is proposed a method for forming a shallow unevenness by compression bonding, through embossing, a low melting point component of a filament nonwoven fabric containing the low melting point component as a sheath component and defining a basis weight and bending resistance (e.g., see Patent Literature 4). Further, there is proposed a method of integrating a filament layer containing a low melting point component as a sheath component and a layer obtained by mixing a filament containing a low melting point component and a filament containing a high melting point component, subjecting the stacked material to embossing, and further adjusting the thickness thereof by calendering (e.g., see Patent Literature 5). There is proposed a method for improving pleatability and pleat retention property by mixing thick composite fibers containing a low melting point component and thin fibers in order to decrease a thermobonding component, subjecting the mixed fibers to embossing, and constituting a frame function with the thick thermobonding fibers (e.g., see Patent Literature 6). As a method for decreasing a bonding component, there are also proposed many methods using petal parts of those having petal type cross sections as thermobonding components (e.g., see Patent Literatures 7 to 12). These methods have a problem of deterioration in durability caused by relaxation attributed to the glass transition temperature of the low melting point component.
As a different method for obtaining a nonwoven fabric with high rigidity, there is proposed a method for three-dimensionally interlacing a filament nonwoven fabric containing no low melting point component by needle punching, impregnating the nonwoven fabric with a binder resin, and fixing the crossing points of the fibers with the resin (e.g., see Patent Literature 13). This method improves rigidity, but has problems that the filtration performance is lowered because of pores by needle punching and further that the binder resin is dropped from the crossing points by outside force during pleating or repetitive use.
As described above, there are proposed methods for improving pleatability or enhancing pleat retention property while keeping the filter performance; however, a problem for obtain a nonwoven fabric with high rigidity that is suitable as a filter having a pleat retention property and durability even in a heating atmosphere has not been yet solved.